Pipelines are the most economically viable method of transporting fluid assets, most commonly oil and gas, but other types of pipeline also exist. A vast pipeline infrastructure exists today responsible for gathering, transporting and distributing these natural resources, with over three quarters of a million kilometers of oil and gas pipelines in the US alone. The continuing proper operation of these pipelines is of paramount importance, and failures carry massive economic loss, environmental impact and potentially catastrophic physical damage also.
Significant efforts are therefore made to maintain, monitor and inspect pipelines. The sheer size of many pipeline networks however, and the fact that many kilometres of pipelines consist of underground or sub-sea installations makes effective and efficient monitoring a difficult problem.
It has been proposed to use fibre optic distributed acoustic sensing to monitor pipelines. International Patent Application Publication WO2010/020796 describes that tens of kilometres of pipeline can be monitored by deploying an optical fibre along the length of the pipeline and interrogating the optical fibre with radiation to provide a fibre optic distributed acoustic sensor. This application teaches that by monitoring the acoustic response of the pipeline to an acoustic stimulus a condition profile of the pipeline can be obtained. The stimulus may be a stimulus deliberately introduced for the purposes of obtaining a condition profile or it could be produced during normal operation of the pipeline, for example as a result of a “pig” travelling through the pipeline.
Various inspection and maintenance tools may be inserted into the pipeline and carried through the pipeline by the pressure of the fluid therein. There are a variety of different objects, commonly referred to as “pigs” that may be employed. A simple cleaning pig may comprise an object shaped to brush or scrape the inner walls of the pipeline as it passes to provide a cleaning action. An intelligent monitoring pig may comprise various sensors to perform various monitoring tasks and an onboard processor and can be a very expensive, very complex instrument.
Use of a pig for inspection or cleaning is commonly referred to as pigging. Pigging is often carried out with largely uninterrupted fluid flow through the pipeline and thus is advantageous in that pipeline flow need not be stopped to perform routine inspection and maintenance.
WO2010/020796 describes that a sudden leak or crack in the pipeline may result in a detectable pressure pulse and that detection of the occurrence of such a spontaneous pulse may be used as part of leak detection and location.
The use of fibre optic distributed acoustic sensing for pipeline monitoring such as taught in WO2010/020796 therefore provides a very useful and convenient way of monitoring large sections of pipeline. The present invention relates to further improvements in relation to monitoring of conduits.